High-temperature process improvements using helium under regulated pressure

ABSTRACT

A method for minimizing unwanted ancillary reactions in a vacuum furnace used to process a material, such as growing a crystal. The process is conducted in a furnace chamber environment in which helium is admitted to the furnace chamber at a flow rate to flush out impurities and at a predetermined pressure to achieve thermal stability in a heat zone, to minimize heat flow variations and to minimize temperature gradients in the heat zone. During cooldown helium pressure is used to reduce thermal gradients in order to increase cooldown rates.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority from U.S. Provisional PatentApplication Ser. No. 61/239,228 filed Sep. 2, 2009 for High-TemperatureProcess Improvements Using Helium Under Regulated Pressure.

BACKGROUND OF THE INVENTION

Field of the Invention

This invention generally relates to the processing of materials in ahigh-temperature operating environment. More specifically this inventionrelates to the production of crystalline material in high-temperaturevacuum furnaces.

Description of Related Art

Although the following discussion relates to the production of crystalsin a high-temperature vacuum furnace, it will be apparent that themethods and apparatus disclosed herein are applicable to anyhigh-temperature operating environment used for the production ofvarious products including, but not limited to, the production ofvarious glasses, amorphous materials, multi-crystalline ingots, such assilicon ingots, and thin films.

A process for producing crystals in an environment, such as found in ahigh-temperature vacuum furnace, can produce any or all of three generalcategories of unwanted ancillary or secondary reactions that can affectthe final crystal quality. They are: (1) high-temperature chemicalreactions, (2) a decomposition of unstable compounds and (3) sublimationor vaporization of certain elements.

High-temperature chemical reactions typically involve carbon orrefractory metals such as molybdenum and tungsten, which are often usedin the construction of high temperature furnaces. If such reactionsoccur, they can degrade the furnace. Examples of reactions involvinggraphite-containing furnaces include:

C(s)+2Mo(s)→Mo₂C   (1)

and

H₂O(g)+C(s)→CO+H₂(g)   (2)

The carbon monoxide can then react with any refractory metals andthereby form carbides and volatile species.

Molybdenum crucibles also react with Al₂O₃ at high temperatures invacuum to form:

Al₂O₃(s)+Mo(s) MoO₂(g)+AlO(g)+Al(g)   (3)

and other compounds as well through similar reactions.

Silicon in a silica crucible operating at a melt temperature of 1412° C.and at a pressure below 10 Torr reacts to form gaseous SiO by thereaction:

SiO₂(s)+Si(s)→2SiO(g)   (4)

At about 1400° C. gaseous SiO reacts with carbon at pressures below 10Torr to produce highly reactive carbon monoxide (CO) and SiC. That is:

2C(s)+SiO(g)→SiC(s)+CO(g)   (5)

Silicon carbide can seriously degrade the quality of silicon grown fromthe melt.

Also, CO reacts with silicon below 10 Torr to form carbon and siliconmonoxide according to:

CO(s)+Si(s)→C(s)+SiO(g)   (6)

Such a reaction can lead to an unwanted deposition of SiO gas ontosurfaces in the colder regions of the furnace. In addition, siliconcarbide can seriously degrade the quality of silicon grown from themelt.

At elevated operating temperatures certain compounds become unstable anddecompose. For example:

2MgO(g)→2Mg(s)+O₂(g)   (7)

whereby free oxygen can react with both the materials used in thefurnace itself and with the material being processed in the furnace toform oxides.

As another example, spinel decomposes to magnesium oxide and aluminumoxide according to:

MgAl₂O₄(s)→MgO(g)+Al₂O₃ (s)   (8)

where MgO reacts as described above.

Sublimation, or vaporization, occurs when certain elements or compoundsare elevated to high temperatures. As known, all metals and somerefractory materials are prone to vaporize or sublime at hightemperatures. Graphite will sublime at high temperature into carbonvapor. For example, graphite will sublime into carbon vapor above 2200°C. . The carbon vapor can react with a crucible and contaminate thecrucible contents.

In crystal manufacturing processes, ancillary reactions, such asreaction (1) above and sublimation, can produce gaseous species. Asknown, such gas can be captured in the material being processed causinga crystal, for example, to have imperfections, such as inclusion orbubbles, which produces undesirable light scatter.

In high-temperature environments, heating elements are also susceptibleto the existence of “hot spots,” which are due to variations inresistivity in the heating element or power source variations, or “coldspots,” which are caused, for example, by leaky insulation. Duringprocessing, hot and cold spots can result in non-uniform ornon-symmetrical crystal growth. During crystal growth and cooldown, hotspots and/or cold spots produce thermal stress gradients in the crystalthat can cause stress defects including dislocations that cause latticedistortion and/or cracking. As known, the existence of fine particles ofprevious reaction products or of furnace construction materials, such asgraphite felt or moisture, that react inside a heat zone can degradefurnace performance and even obscure viewing through viewports.

What is needed is a process and control thereof that minimizes unwantedancillary reactions and unwanted temperature gradients during processingin high-temperature environments.

SUMMARY

In accordance with one aspect of this invention, a method of producing acrystalline material is disclosed that comprises the steps of a) loadinga crucible having melt stock and an optional seed crystal into a heatzone of a furnace; b) evacuating the heat zone of the furnace to anoperating pressure value; c) heating the heat zone of the furnace to atleast partially melt the melt stock; d) further heating the heat zone ofthe furnace to a maximum temperature to fully melt the melt stock andoptionally to partially melt the seed crystal; e) growing thecrystalline material from the fully melted melt stock and optionallyfurther from the partially melted seed crystal by cooling the heat zone;and f) removing the crystalline material from the furnace. The methodfurther comprises the steps establishing a flow rate of at least onenon-reactive gas into the heat zone of the furnace followed byestablishing a pressure of the at least one non-reactive gas in the heatzone of the furnace above the operating pressure, which occurs prior tothe step of growing the crystalline material.

In accordance with another aspect of this invention, a furnace forproducing a crystalline material is disclosed that comprises a heatzone; a vacuum pump assembly connected to the furnace that maintains avacuum in the heat zone; at least one heater surrounding the heat zonethat provides heat to the heat zone; a non-reactive gas system connectedto the furnace that supplies at least one non-reactive gas into the heatzone; and a non-reactive gas regulating system connected to the furnaceand to the vacuum pump assembly that establishes and maintains a flowrate and a pressure of the non-reactive gas in the heat zone. Thefurnace may further comprise a heat exchanger.

In accordance with another aspect of this invention, a method isdisclosed for stabilizing the environment in a high-temperature vacuumfurnace for producing a crystal or ingot wherein production is subjectto at least one unwanted temperature and pressure dependent ancillaryreaction, said method comprising: A) establishing a reaction pressurevalue dependent upon the reaction vapor pressure for each of the atleast one ancillary reaction, B) directing gas through the heat zonewherein the gas has the characteristics of being a non-reactive in theheat zone environment and of small molecular size, high specific heatand high heat conductivity, and C) regulating the pressure of thenon-reactive gas in the heat zone at the established reaction pressurevalue.

BRIEF DESCRIPTION OF THE DRAWINGS

The appended claims particularly point out and distinctly claim thesubject matter of this invention. The various objects, advantages andnovel features of this invention will be more fully apparent from areading of the following detailed description in conjunction with theaccompanying drawings and in which:

FIG. 1 is a block diagram that depicts the application of this inventionto a first type of furnace;

FIG. 2 is a block diagram that depicts the application of this inventionto a second type of furnace;

FIG. 3 is a basic flow diagram that outlines the methodology for processcontrol according to this invention that can be applied to variousfurnaces including those in FIGS. 1 and 2;

FIG. 4 presents an analysis of the quality of a crystal using a firstcomparative process ;

FIG. 5 presents an analysis of the quality of a crystal using a secondcomparative process;

FIG. 6 is a basic flow diagram for a process control methodologyincorporating this invention for the production of sapphire crystals;and

FIG. 7 presents an analysis of the quality of a sapphire crystal that isachieved by using this invention.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

Two specific furnaces described in detail are a material processingfurnace in FIG. 1 and a heat exchanger method (HEM) furnace in FIG. 2.It will be apparent, however, that this invention can also be adaptedfor growing ingots by directional solidification or crystals inBridgman, Stockbarger, Thermal Gradient Freeze, Top Seeded Kyropolous,Thermal Gradient Technique and other furnaces where temperaturestability and uniformity and control are important.

Material Processing Furnace

Now referring to FIG. 1 a furnace system 10 adapted for processingmaterials in accordance with this invention includes a vacuum tightfurnace chamber 11. A vacuum pump assembly 12 evacuates the interior ofthe furnace chamber 11 and is shown schematically as comprising a vacuumpump 12P, a main vacuum gauge 12G for providing measurements for avacuum controller and a vacuum valve 12V. Such vacuum systems are knownin the art.

In this embodiment, insulation 15 in the furnace chamber 11 forms aninsulated heat zone 16. The insulation 15 can be composed of agraphite-based material, such as graphite felt or other refractorymaterial. The heat zone 16 that can be formed by any of a number ofknown means or structures used in various furnace designs.

The heat zone 16 in FIG. 1 includes a graphite resistance heater 17 withleads 20 that extend to a power source in a furnace control cabinet 21.At least one pyrometer 22 measures the process temperature through awindow and port, such as window 23A and port 23B, to provide atemperature input signal for process control. In this embodiment aproperly conditioned crucible 25 positioned in the heat zone 16 containsa mix of materials that constitute a melt stock. The resistance heater17 surrounds the crucible 25. Such heaters and controls are known in theart.

In this embodiment, the furnace system 10 is adapted to incorporate anon-reactive gas system 30. The gas system 30 supplies a non-reactivegas into the volume defined by the furnace chamber 11 including the heatzone 16. The gas system 30 includes a supply tank 31 or other source. Aflow controller 32 includes a flow monitor 33 and controls the gas flowrate into the furnace chamber 11. Conventional flow control systems canbe adapted to perform this function.

The gas system 30 supplies the non-reactive gas into the heat zone 16,preferably through a port 40 at the base of the furnace chamber 11 withvacuum valve 42 closed. A pressure regulator 41 controls the gaspressure in the furnace chamber 11 by bleeding gas to the vacuum pumpassembly through a vacuum line 43 in a controlled manner. As will bedescribed, at some point during operations and in accordance with thisinvention, the main vacuum valve 12V is closed and the valve 42 is opento provide a pressure regulated exhaust path from the vacuum chamber 11to the vacuum pump 12. As a result, the pressure in the heat zone 16 isallowed to increase to, and thereafter be maintained at, a predeterminedpressure. Systems for providing gas at variable flow rates and at apredetermined pressure are known in the art.

Heat Exchanger Method (HEM) Furnace

FIG. 2 depicts a HEM (Heat Exchanger Method) furnace 50. Variations ofsuch a furnace are described in U.S. Pat. Nos. 3,653,432 and 3,898,052and 4,256,530 and 4,840,699 and 7,344,596. In FIG. 2, the furnace system50 includes a vacuum tight furnace chamber 51. A vacuum pump assembly 52evacuates the interior of the furnace chamber 51 and is shown ascomprising a vacuum pump 52P, a vacuum gauge 52G for providingmeasurements for vacuum control and a main vacuum valve 52V as known inthe art. Insulation 55 in the furnace chamber 51 defines a heat zone 56which can be constructed in accordance with any number of variations.

The heat zone 56 is bounded by a graphite resistance heater 57 withleads 60 that extend to a power source typically associated with afurnace control cabinet 61. At least one pyrometer 62 measures theprocess temperature through a window and port, such as window 63A andport 63B, to provide a temperature input signal for process control.Heat exchanger 64 can comprise a closed-end refractory metal tube withan internal tube to inject a coolant gas, particularly helium, atcontrolled rates to extract heat from the bottom center of the crucible.The heat exchanger 64 supports a properly conditioned crucible 65 thatis surrounded by the resistance heater 57. The crucible 65 contains amix of melt stock materials that will produce a final crystal.

In FIG. 2, the heat exchanger 64 provides selective cooling byconnection to a helium cooling system 70 with a helium supply tank 71 orother source. A helium recirculation pump 72 pumps helium gas to a massflow controller 73 or valve that controls the gas flow rate into theheat exchanger 64. The combination of the variable temperature controlof the heater 57 and variable helium flow control to the heat exchanger64 enables heat to be extracted directionally from the seed and melt inthe bottom of the crucible for good directional solidification orcrystal growth. That is, the heat exchanger 64 produces a temperature inthe bottom center of the crucible at the seed crystal for single crystalgrowth by increasing the helium flow to reduce the heat exchangertemperature in a controlled manner to produce a desired solid-liquidinterface shape and growth rate. Single crystal growth thereby occursunder highly controlled liquid and solid temperature gradients whichefficiently segregate impurities into the liquid as the solid-liquidinterface advances. Crystal growth methods where crystal growth from thebottom of the crucible like Bridgman, Stockbarger and othercrystal-growth and ingot-growth methods where crystal growth occursinside the crucible will benefit by using a heat exchanger 64 or otherequivalent device with regulated helium flow to achieve the best processcondition for each type of furnace and/or material being processed.

In this embodiment, the furnace system 50 is adapted to incorporate ahelium gas, or other non-reactive gas, system 80. The gas system 80comprises a supply tank 81 or other source. A mass flow controller 82and flow monitor 83 control the rate at which the gas flows through aconduit 84 to be admitted into the furnace chamber 51, preferably at thebottom.

Like the apparatus in FIG. 1, the furnace system 50 operates under avacuum initially established when the vacuum system 52 evacuates theheat zone 56 through the vacuum valve 52V. A second evacuation path isprovided through a control valve 84 and pressure regulator 85 thatexhausts through a valve 86 into the vacuum pump system 52. At somepoint during operations and in accordance with this invention, the mainvacuum valve 52V closes and the valve 84 opens to provide a controllablegas pressure within the heat zone 56.

Process Control

In accordance with one aspect of this invention, a method of producing acrystalline material is disclosed that comprises the steps of providinga crucible having a melt stock into a heat zone of a furnace andevacuating the heat zone of the furnace to an operating pressure value,which is typically less than 1 Torr. Optionally, the crucible may alsocontain a seed crystal. In this method, the heat zone of the furnace isheated to at least partially melt the melt stock and then further heatedto a maximum temperature to fully melt the melt stock and optionally topartially melt the seed crystal, if used. The method further comprisesthe step of growing the crystalline material from the fully melted meltstock and optionally further from the partially melted seed crystal bycooling the heat zone. The crystalline material can then be removed fromthe furnace.

The method of the present invention further comprises the stepsestablishing a flow rate of at least one non-reactive gas into the heatzone of the furnace followed by establishing a pressure of the at leastone non-reactive gas in the heat zone of the furnace above the operatingpressure, which occurs prior to the step of growing the crystallinematerial, including prior to the step of melting the melt stock. Inaccordance with this invention and as previously discussed, processcontrol is achieved by the use of helium or any other gas that isnon-reactive in the furnace environment and that has a small molecularsize, high specific heat and high heat conductivity. Non-reactive gasesthat are candidates for use in accordance with this invention includeargon, helium and nitrogen and, in oxygen-free environments, hydrogen.

In a preferred embodiment, helium is selected because it has significantadvantages. It does not react at high temperatures. It has better heatconductivity and higher specific heat than argon and nitrogen, andtherefore it provides better temperature stability. It has a smallermolecular size and better flow properties than argon. This enables goodconvective heat transfer and provides better diffusion of gas out ofmaterials in a vacuum. Although hydrogen has better heat conductivityand smaller molecular size than helium, it is a reducing gas andexplosive when in contact with oxygen, particularly at the highoperating temperatures and conditions of these processes and cannot beused for processing most materials. Therefore, the following processcontrol is couched in terms of the use of helium gas. However, as willnow be apparent argon or nitrogen can be used for some processes whereheat flow and/or temperature uniformity are not so critical to providesimilar effects relative to helium.

As discussed in more detail above, processes for producing crystals in ahigh-temperature vacuum furnace can produce a variety of unwantedancillary or secondary reactions that can affect final crystal quality.Each unwanted ancillary reaction has two properties, namely: (1) areaction temperature and (2) a reaction vapor product, which can be agas or solid. The reaction temperature is the minimum temperature atwhich an unwanted ancillary reaction occurs. If the reaction product isgaseous, the reaction vapor pressure, or reaction pressure, is thatpressure above which the ancillary reaction is suppressed. Specificvalues for reaction temperatures and vapor pressures are known for thedifferent reactions encountered in the above-identified and otherfurnace systems.

Decomposition reactions can often be suppressed by imposing an inert gaspressure greater than or equal to the total vapor pressure of thedecomposition products. For example, carbonate decomposition reactionsproceed at significant rates only when the decomposition vapor pressureof CO₂ is greater than the total pressure, even when the partialpressure of CO₂ is less than equilibrium. Similarly, the decompositionof Si₃N₄ can be suppressed if a total pressure greater than the vaporpressure of Si₃N₄ is maintained.

If there is concern with only one unwanted ancillary reaction, areaction threshold can be established for the reaction temperature andvapor pressure for that unwanted ancillary reaction. If multipleunwanted ancillary reactions are involved and if the operationaltemperature is higher than the threshold reaction temperature, thepressure must be higher than the vapor pressure of each involvedancillary reaction. See Schmid, Origin of SiC Impurities in SiliconCrystals Grown from the Melt in Vacuum, J. Electrochem. Soc. 126 (1),935 (1979).

A vapor pressure threshold is selected to be greater that the largestvalue of the reaction vapor pressures and a minimum helium flow rate isestablished to flush out impurities and contaminants. As will beapparent to those of ordinary skill in the art, the reaction temperatureand pressure thresholds involve kinetics and are determined bythermo-chemical calculations, experience and experimentation for anyspecific reaction. In the following discussion, the analysis ofpotential unwanted ancillary reactions will produce an establishedreaction temperature value and an established reaction pressure valuewhich are used as critical values for the process.

Now referring to FIG. 3, a process 100 of this invention begins with atypical series of preliminary steps 101 through 104 that are known inthe art. In this specific embodiment step 101 represents the process ofpurging the furnace to assure that it is clean. At the end of step 102the crucible is in the heat zone of the furnace and contains thematerials to be processed. Step 103 represents an evacuation of the heatzone and bakeout temperature to remove any volatile impurities. Afterconditioning in step 104 the temperature in the heat zone begins to rampup to process the contents in the crucible.

When, in step 105, it is determined that the temperature has ramped tothe established reaction temperature, in step 106 helium gas flow isinitiated to bring the pressure within the heat zone to the establishedflow rate and reaction pressure to minimize gas evolution. The heliumgas flow rate is sufficient to maintain a precise regulated pressure andflush out contaminants that could contaminate the heat zone and thematerials being processed in the heat zone. Typically a vacuum furnaceis operated in the range of 5 to 250 Ton of helium gas depending on thematerial being processed. Maintaining a pressure in this range isreadily achieved with conventional systems with the helium flow into andout of the furnace chamber.

The flow rate should assure that the helium is heated sufficiently whenit enters the heat zone from the bottom of a furnace chamber. As known,increased heat transfer in the heat zone from hotter regions to coolerregions will increase to minimize temperature gradients thus achievinggreater temperature uniformity in the heat zone. The overall heattransfer from the heat zone to cooler portions of the furnace chamberwill also increase. As will be apparent, any such increase in heattransfer will require the heater control to supply more power. Helium,with its good heat conductivity and good flow characteristics, enhancesheat transfer in the heat zone and increases heat flow through theinsulation to the inside wall of the chamber. This, in turn, requireshigher power. Heat flow can be increased or decreased with increases ordecreases in pressure. For any given process, the minimum pressureshould be selected to provide good thermal uniformity in the heat zonewhile minimizing the power requirements and assuring that the flow rateis sufficient to sweep impurities out of the heat zone. The good flowcharacteristics cause helium to flow through openings from the inside tooutside of the heat zone. It is important to minimize openings out ofthe heat zone since the openings act as chimneys, and this increases therequired power and causes temperature variations. Ideally, any openingsshould be minimized and symmetrical in the heat zone.

Once the gas flow reaches its established parameters in step 107 theheat zone temperature increases while controlling the helium pressurewith a regulator that adjusts the helium flow to the vacuum system.

Step 110 represents the continued operation for growing the crystalwhile maintaining the helium pressure and flow rate. The pressureregulators 41 and 85 in FIGS. 1 and 2 and/or a pressure transducer withrelated controls provide the necessary pressure control.

During step 110, as an example, there is a strong reduction potential ingraphite resistance furnace chambers. If there were an oxygen leak,carbon vapors could react with oxygen to form carbon monoxide. Carbonmonoxide could then react with and reduce the contents in the cruciblebeing processed.

In certain processes involving aluminum oxide, the crucible may becomposed of molybdenum. This will cause a reduction of aluminum oxideinto suboxides and form molybdenum oxide. Helium pressure applied inaccordance with this invention minimizes the reduction of aluminum oxideand reduces light scatter in the resulting crystal.

It is believed that the helium atmosphere also suppresses vaporizationof metals, refractory materials, graphite or carbon at hightemperatures. These vapors can react to deteriorate furnace components,contaminate and/or reduce the materials being processed. It appears thatmaintaining helium pressure stabilizes heat zone temperature byminimizing temperature gradients and fluctuations that are sources ofdislocations and lattice distortion.

When, in step 111, it is determined that the growth has been completed,cooldown is initiated in step 112. In accordance with this inventionthis cooldown occurs at an increased helium pressure, typically byincreasing the helium pressure above that used during step 110 (i.e.,the established reaction pressure value). Increasing the pressurestabilizes the temperature in the heat zone by minimizing hot and coldspots. It has also been found that increasing the pressure of the heliumor other gas increases the heat loss to the furnace's water-cooledchamber. This reduces the cooldown time of step 113 by minimizing thegradients in the heat zone which increases the likelihood of successfulannealing and may reduce the overall cycle time.

With this understanding of the process of this invention as describedabove, it will be helpful to describe the benefits of this invention bydisclosing two specific examples of a process for growing a sapphirecrystal without the benefit of this invention and then disclosing aspecific example of a process incorporating this invention and thecrystal produced thereby.

As known, sapphire crystals are useful as substrates in the productionof light emitting diodes. As also known, sapphire crystals used for thispurpose must have a lattice distortion resulting from high dislocationdensity and light scatter site density that are below predeterminedlevels.

FIG. 4 is a comparative example of a sapphire boule produced in a priorart vacuum furnace of the type shown in FIG. 2, but without the flow ofa non-reactive gas. This yielded a sapphire boule as shown in FIG. 4which is based upon a cross-section along the C axis of the boule. Thecross-hatched portion of FIG. 4 depicts the small portion of that boulethat meets the lattice distortion and scatter site requirements for thelight-emitting diode application.

FIG. 5 is another comparative example of a sapphire boule produced by anexperiment in which the process for producing the boule of FIG. 4 wasmodified by adding helium into the furnace but without pressure control.The cross-hatched area represent that portion of the boule that did meetthe lattice distortion and scatter site specifications and thereforecould be used in the manufacture of light-emitting diodes. Byinspection, this is a small percentage of the total boule volume. Italso was found that the process cycle for the production of this boulewas longer than the process cycle for the boule in FIG. 4.

FIG. 6 is a flow chart of a process for producing a sapphire crystalfrom alumina in accordance with this invention. Step 201 represents thephysical requirements for (i) centering a seed in the crucible so thatit remains in place, (ii) placing the crucible in a furnace and (iii)adding alumina melt stock to the crucible.

Step 202 is an initial stage involving the evacuation and initialheating of the furnace such that any major impurities can be removed byvacuum. Concurrently, a coolant gas, such as helium, beings to flowthrough a heat exchanger, such as the heat exchanger 64 in FIG. 2. Next,at step 203, the temperature in the heat zone continues to ramp meltingis observed. The ramp can be determined and controlled by any number ofknown processes.

In this specific example, this determination is made by monitoring theemissivity from the top of the crucible. When melting occurs, theemissivity changes. Emissivity change detection devices are known in theart and they provide a variety and they announce the onset of melting isa variety of ways. When this occurs, the process shifts into a meltstage.

At step 205 in the melt stage, helium gas flow through the furnacebegins at a specified rate while the main vacuum valve, such as the mainvacuum valve 52V in FIG. 2, is open to purge or flush the heat zone.This continues for a sufficient time to complete the flushing ofimpurities.

When the operations of step 205 are complete, in step 206 the mainvacuum valve closes. Then helium flowing into the heat zone is nowremoved through a vacuum proportional valve that controls the pressurein the heat zone . The pressure is in the range of 5-50 Torr, preferably10-30 Torr, and the flow rate is not greater than 1.0 SCFH, such as 0.05to 1.0 SCFH including 0.1 to 0.5 SCFH. In accordance with this inventionand as shown in step 207, this helium pressure and flow are controlledthroughout the remainder of the cycle. It is believed that controllingthis pressure and flow rate assures that unwanted ancillary reactionsarc suppressed and that impurities arc flushed from the heat zone.

In step 206 the temperature is increased to a maximum value (e.g., a“seeding temperature”) at which the melted alumina penetrates the seed.The rate of temperature increase and the duration of the process at thismaximum temperature value is dependent upon a number of factors andexperience.

At step 211 a growth stage begins by reducing the temperature in acontrolled manner. Specifically, the helium flow through the heatexchanger (64 in FIG. 2) increases thereby removing heat from thecrucible and its molten contents. The optimal rate of change of heliumflow in the heat exchanger is determined experimentally, but oncedetermined remains essentially reproducible for a given furnace. Thisheat extraction enables the directional solidification to occur therebyto produce the final crystal and continues until the temperature in theheat zone reaches an annealing point temperature. When the annealingtemperature has been reached in step 212, the crystal is annealed.. Instep 213 the furnace is isolated and the helium continues to flow tobackfill the heat zone. Thereafter the resulting crystal can be removed.

EXAMPLE

FIG. 7 is a cross-section along the C-axis of the boule produced usingthe process of FIG. 6. The lined area represents that portion of theboule that met the minimum lattice distortion and scatter sitespecifications and therefore could be used could be used in themanufacture of light-emitting diodes. There is a significant increase inthe useful volume over that shown in FIGS. 4 and 5. Moreover the usefulvolume in this boule exceeded both the lattice distortion and scattersite requirements. The cycle time for this improved product was aboutthe same as the cycle time for the process run of FIG. 4.

From experience it is known that different furnaces may have differentcharacteristics. Certain of those characteristics can result in changesduring the growth stage and can adversely impact the quality of thefinal crystal. When it is determined that such conditions exist andcould be overcome by modifying the constant nature of the heat transferduring the growth stage, temperature corrections can be made withoutchanging the power supply settings. Specifically, it has been found thatvarying the helium pressure in the heat zone has the effect of changingthe temperature in the heat zone with very good control. Thus whilenormally the established helium pressure may be constant, the pressurecan be intentionally varied as required to effect such changes whilestill achieving the benefits of this invention, particularly during thegrowth stage.

As will now be apparent this invention provides many advantages byenabling the production of crystals, such as sapphire crystals, invacuum furnaces whereby a significant portion of the grown crystal canmeet or exceed requirements for lattice distortion and scatter in thecrystal. A furnace that incorporates this invention provides an inertenvironment that minimizes the unwanted reactions typically encounteredin high temperature environments such as unwanted reactions,decomposition, sublimation and vaporization. The use of this inventionenables a high temperature process to operate at higher temperatureswith a more uniform temperature in the heat zone. Higher pressures ofthe non-reactive gas provide better heat transfer for annealing crystalsand for cooling faster by minimizing temperature gradients caused by hotand cold spots in the furnace. Minimizing hot spots can be achieved byincreasing helium pressure within the zone to improve heat flow.Maintaining non-reactive flow rates also minimizes the reaction productsthat remain in the heat zone.

This invention has been disclosed in terms of certain embodiments. Itwill be apparent that many modifications can be made to the disclosedapparatus and process steps without departing from the invention.Therefore, it is the intent of the appended claims to cover all suchvariations and modifications as come within the true spirit and scope ofthis invention.

What is claimed is: 1-23. (canceled)
 24. A furnace for producing acrystalline material comprising: a heat zone; a vacuum pump assemblyconnected to the furnace that maintains a vacuum in the heat zone; atleast one heater surrounding the heat zone that provides heat to theheat zone; a non-reactive gas system connected to the furnace thatsupplies at least one non-reactive gas into the heat zone; and anon-reactive gas regulating system connected to the furnace and to thevacuum pump assembly that establishes and maintains a flow rate and apressure of the non-reactive gas in the heat zone,
 25. The furnace ofclaim 24, further comprising a heat exchanger.
 26. The furnace of claim24, wherein the non-reactive gas system supplies the non- reactive gasto the heat zone from beneath the furnace.
 27. The furnace of claim 24,wherein the non-reactive gas system comprises a supply tank, a flowcontroller, and a flow monitor.
 28. The furnace of claim 24, wherein thenon-reactive gas regulating system comprises a pressure regulator and apressure monitor.
 29. The furnace of claim 28, wherein the pressurecontroller is a proportional valve. 30-34. (canceled)